These studies aim: 1) to isolate lupus anticoagulants by affinity chromatography on PS-PA Sepharose and by adsorption to liposomes, characterize their antigenic specificity and molecular heterogeneity and study their interaction with platelet membrane. 2) to study the orientation of platelet phospholipids using lupus anticoagulants, radioactive TNBSA and lactoperoxidase catalysed iodination of membrane phospholipids. 3) to identify the platelet membrane component(s) that serves as the receptor for factor Xa-Va complex. 4) to explore the role of platelets and isolated platelet membranes (by nitrogen decompression technique) in factor X activation. 5) to study the interactions of antibodies with phospholipid specificity occurring in humans and animals (MOPC and TEPC tumors in BALB/c mice) with platelet and other cell membranes.